1. Field of the Invention
The present invention relates to an ink-jet printing head arranged to eject or deliver droplets of an ink from respective nozzles communicating with respective pressure chambers, upon pressurization of ink in the pressure chambers.
2. Discussion of Related Art
U.S. Patent Application Publication No. 2002/0105567 (in particular, FIGS. 7 and 8, and paragraphs [0055] and [0056]) which corresponds to JP-A-2002-273894 discloses an ink-jet printing head including a flow-passage unit and an actuator unit which are laminated on each other. The flow-passage unit has nozzles and pressure chambers which are formed so as to be open in the respective opposite major surfaces such that the nozzles are held in communication with the respective pressure chambers. The actuator unit is constructed and operable to apply pressure to ink in each pressure chamber. The flow-passage unit and the actuator unit are superposed on each other such that the pressure chambers are located adjacent to the actuator unit. In the ink-jet printing head of this type, a drop of the ink is ejected from each nozzle when the volume of the corresponding pressure chamber is changed by deformation of a corresponding active portion of the actuator unit in the direction of lamination of the flow-passage and actuator units, which deformation takes place based on a piezoelectric effect produced at the active portion.
Each of the pressure chambers formed in the ink-jet printing head disclosed in the above-identified publication U.S. Pat. No. 2002/0105567 has a generally rectangular shape that is elongate in one direction, as seen in a plane parallel to the opposite major surfaces of the flow-passage unit. The longitudinal direction of each pressure chamber is perpendicular to the longitudinal direction of the ink-jet printing head, that is, parallel to the direction of width of the head. The pressure chambers are arranged adjacent to and spaced apart from each other in the longitudinal direction of the ink-jet printing head, in at least one row extending in the longitudinal direction, so that the nozzles are equally spaced apart from each other at an extremely small spacing pitch in the longitudinal direction of the head. The pressure chambers have a depth of 50 μm–150 μm that is equal to the thickness of a metal plate through which the pressure chambers are formed. Such pressure chambers arranged in an ink-jet printing head as disclosed in the above-identified publication typically has a length of about 3.8 mm and a width of about 250 μm. Such an actuator unit as disclosed in the above-identified publication is typically driven with a drive voltage of 21.5 V, at a maximum drive frequency of 18 kHz, and an ink droplet is ejected from the nozzle at an ejection velocity or a delivery rate of about 6–7 m/sec.
An ink-jet printing head is generally operated with a so-called “fill-before-fire” action to eject an ink droplet from each nozzle. The fill-before-fire action involves deformation of the active portion of the actuator unit so as to initially increase the volume of the corresponding pressure chamber from a nominal value in a non-energized state of the active portion. An increase in the volume of the pressure chamber due to the deformation causes a pressure wave to be generated within the pressure chamber. The pressure wave propagates through the pressure chamber in its longitudinal direction. When the pressure within the pressure chamber has been raised back to an original positive value, the active portion is de-energized to restore the pressure chamber to the original shape, and to reduce the volume of the pressure chamber back to the nominal value, thereby pressurize the ink within the pressure chamber. The fill-before-fire action involves mutual superimposition of the pressure wave and the pressure application due to the deformation of the active portion back to the original shape, permitting the ink droplet to be ejected at a sufficiently high velocity, with a comparatively low drive voltage applied to the active portion of the actuator unit.
In an effort to meet a recently growing need for increasing the printing speed of the ink-jet printing head, it has been attempted to increase the maximum drive frequency of the actuator unit. In the fill-before-fire action, the maximum drive frequency depends upon the time required for reciprocal propagation of the pressure wave through the pressure chamber in its longitudinal direction, namely, depends upon the length of the pressure chamber, as is understood from the foregoing explanation of the fill-before-fire action. Accordingly, the maximum drive frequency can be increased by reducing the length of the pressure chamber.
However, a reduction in the length of the pressure chamber without changing the drive voltage of the active portion of the actuator unit causes the ink ejection velocity to be accordingly lowered, giving rise to a risk of deterioration of a quality of an image formed by the ink droplets. The ink ejection velocity can be increased by increasing the drive voltage of the actuator unit (to 25 V, for example). However, an increase in the drive voltage results in not only an increase in the cost of manufacture of the associated components such as electric circuits and driver ICs, but also an increase in the amount of heat generation from the actuator unit, which requires the provision of additional components for dissipating the generated heat. While the ink ejection velocity can also be increased by reducing the diameter of each nozzle, this solution undesirably results in a decrease in the amount of the ink to be ejected from the nozzle.
Thus, the ink-jet printing head known in the art is not capable of ejecting ink droplets at a sufficiently high velocity, at a sufficiently high drive frequency and with a sufficiently low drive voltage. For enhancing the quality of the image formed by the printing head, it is desired to increase the ink ejection velocity as much as possible.